This invention relates to telephony and more particularly to an electronic tone ringer for a telephone set.
In a conventional telephone ringing system, an a.c. ring signal, typically 20 hz., produced by a ring generator at a central location from the telephone set, is connected to the tip and ring leads of the subscriber's telephone set. In such a system, the 20 hz. ring signal is used to energize an electromagnetic coil in the telephone set which in turn activates a mechanical clapper for a bell to create the ringing sound of the conventional telephone set.
Such a telephone ringing system with an electromechanical bell ringer suffers several disadvantages. In the first place, driving the electromagnetic coil requires a substantial amount of power thereby limiting the number of telephones that can be driven by a single telephone ring generator. In the case of private exchanges where numerous telephones are connected to a single central switching system additional ring generators may be required, especially in subscriber carrier systems.
Second, the conventional electromechanical bell ringer is heavy in weight because an iron core is required in order for the electromagnetic coil to generate sufficient magnetic flux to drive the bell's clapper.
Third, the electromechanical bell ringer is prone to failures resulting from mechanical wear.
Electronic tone ringers have been proposed in the past, but in most cases these electronic tone ringers utilized a speaker having a paper cone which was fragile and subject to deterioration and damage. Furthermore, such electronic ringers having a speaker often required a transformer to drive the speaker and a heavy magnet as part of the speaker. The use of a transformer with its iron core and magnetic speaker provided very little advantage over the conventional electromechanical bell ringer.
Also because the electronic circuitry could respond instantaneously to a.c. signals on the telephone leads, the electronic ringers also had the annoying tendency to sound in response to transients in the telephone leads caused by dialing pulses or the line being seized prior to ringing. The conventional electromechanical bell ringers were not prone to respond to such line transients because of the magnetic and mechanical time delays inherent in the electromechanical ringer with its magnetic coil and mechanical clapper.
It is, therefore, an object of the present invention to provide an electronic tone ringer which is light in weight, rugged in construction, and economical to build.
Furthermore, it is an object of the present invention to provide an electronic tone ringer requiring low power which can operate directly from the ring signal and which requires no other source of electromotive power.
It is also an object of the present invention to provide an electronic tone ringer which is immune to line transients.
It is a further object of the present invention to provide an electronic tone ringer which is adaptable for use in a two-party telephone system.